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Oxidizer VoC Help
2

Oxidizer VoC Help

Oxidizer VoC Help

(OP)
We have a vapor control system in our process that pulls monomer vapors (mainly acrylics, but some vinyls) to one of two thermal oxidizers. We recently had a stack test performed as part our air-permit, and the results were perplexing. In all prior tests (going back every 4 years to the early 2000's) the destruction efficiency of the oxidizers was >98% for both oxidizers, as required by permit. In the most recent test, efficiency ranged from 95-97%, well below permit requirements.

We have not changed anything regarding the system design since the last test in 2016. The incoming air stream with VoC content is the same flow rate/pressure, and the TO is operated at the same temperature as all previous tests. No change in piping configuration has occurred. While we did not conduct speciation testing this time, the source batches for the VoCs were very similar to previous tests. Incoming ppmv of vapors was the same as previous tests. The test methodology was the EPA-required method for carbon, so that should not be different.

There was, however, one change. The testing on the outlet was conducted in the stack, and this time the test ports were ~15' lower than all previous testing. The oxidizer has an economizer, and the gas stream that leaves the economizer and goes to the stack is still >800 F. I believe we may still be getting destruction at that temperature range. If so, dropping the testing height reduced our available residence time before testing, which could lead to the decreased efficiency.

There is one supporting factor for this. During the test of TO1, we had TO2 cooling down with some dilution air. The outlet of these two thermal oxidizers tie in together into a single stack at the bottom, with the test port ~15' above the top inlet port. During the test (while TO2 had dilution air going), stack temperature was read at ~700F at the testing port, and efficiency was ~95%. We cut the dilution air blower off to TO2. Stack temperature increased to 800F, and efficiency immediately increased to 97%.

Time, temperature, turbulence, and excess O2. Those are the variables that I've read that effect destruction efficiency. However, with a re-test fast approaching, I am searching for any other explanations of why our TOs are suddenly not performing as they have in the past. My initial reason is above, but I wanted to check with the members here to see if you have any insights you could offer.

Any help is appreciated.

RE: Oxidizer VoC Help

I did a lot of work with thermal oxidizers in the 80's. Without a catalyst bed, which can be easily poisoned, the EPA requirement was to heat to 1400F for about one second, as I recall. Reaching this peak temperature, with excess O2, was the most critical factor. At lower temperatures the required residence time goes way up and a catalyst is needed. You should be monitoring the peak temperature and not stack temperature.

Corrosion of sheet metal may cause leaks that allow gasses to bypass the incinerator.

RE: Oxidizer VoC Help

(OP)
We do monitor the temp in the oxidizer - it typically stays around 1340 F. We went up to 1400 F during the test with no discernible change in efficiency.

The oxidizers have been installed 15-18 years. It is possible that the tubesheets in the economizer are leaking, but it would have to be BOTH economizers that have developed leaks in the last couple of years. FYI, they have been run evenly since installation - they alternate weeks on operation.

Could we dye-pen the tube welds to determine if the welds are sound? I don't have any documentation on the economizer, so I don't know the MAWP. As such, I am hesitant to hydrotest it to look for leaks.

RE: Oxidizer VoC Help

Could there be a leak of unburnt waste gas going through TO2 at the time you ran the test on TO1?

To check if the economiser in TO1 is leaking, would there be an economiser bypass line you could use, then isolate the economiser ?

Also pull out and check the burner nozzles - are they corroded - could swirl mixing eff be poorer now. Also, check the TT in the firebox - recently calibrated?

RE: Oxidizer VoC Help

(OP)
There could be a leak that bypasses process air through TO2, sure. However, we still did not have good efficiency after TO2 dilution air was shut down. With no fan trying to pull process air into TO2, there is no motive force for having air leak by.

No, the economizer is bolted directly to the TO outlet. So no hope of using the bypass to get a binary comparison like that. We will have to disconnect the attached piping to inspect it.

We had a burner nozzle guy out who looked at both burners through the sight glass. He said the flames looked good, but I will make sure we inspect the burner nozzle when we inspect the TO next week.

RE: Oxidizer VoC Help

You dont need to pull out the econ bank to see if it is leaking. Once isolated at the external valves, you would see a drop in VoC in the exhaust gas, assuming you still keep the firebox temp the same as previous?

RE: Oxidizer VoC Help

2
(OP)
Thanks for the responses, all. We pulled the process-side piping. The tubesheet on the economizer was cracked and separating, with the tubesheet completely separated from about 12% of the tubes. The area that is cracked/separated is the spot where the hot gases from the combustion chamber first enter the economizer. That zone is experiencing the highest thermal stresses, so it's not surprising that the failure point is there.

We also have dye-pen tested the rest of the tubes that are not obviously cracked and found minor cracks in the welds elsewhere.

Now to test our second TO. I've got a lunch bet riding on it having a similar type failure on its tube-sheet as well.

RE: Oxidizer VoC Help

Thank you for reporting back to us. This has tremendously increased the educational value of this thread. One aside, in my experience this is called a recuperator rather than economizer. An alternative technology to recover heat is a regenerator.

"Regenerators and recuperators are heat exchange systems that recover heat by cycling through heat sinks (regenerators) or through a high temperature metallic heat exchanger (recuperators).

The heat recovered by a regenerator or recuperator is most commonly used to preheat combustion air to a furnace. Regenerators and recuperators may also be attachments to thermal oxidizers that preheat the exhaust air from industrial processes prior to treatment for HAP and VOC contaminants.

Regenerators are more durable and forgiving than recuperators, and may be the only choice when exhaust temperatures exceed the limits of metals, such as found in glass melting furnace applications. The heat sink materials may be brickwork or more sophisticated ceramic products. Recuperator designs often need to address temperature limitations, corrosion issues, pressure drop limitations, and heat recovery levels."

RE: Oxidizer VoC Help

(OP)
All,

We pneumatically leak checked the economizer (or recuperator, if you will) in place at 1 psig. Upon closing the valve, the pressure dropped to 0 within a couple of seconds. All gasket connections were bubble-checked, so the leak had to be internal. Fortunately, this is a removable-bundle type. We pulled the economizer out and lifted the bundle out. A significant crack was found on the floating head side (not sure what to call it, so I'm just calling it the plenum box for now).

We repaired the crack on the plenum box, and again pneumatically tested at 1 psig. The bundle still did not hold pressure, taking ~30 seconds to drop from 1 psig to 0 on the gauge. Throttling the inlet air valve to maintain 1 psig on the bundle, I soaped the tubes. The resulting test pictures are below. I estimate 15-25% of the tubes are leaking at the tubesheets, both on the plenum box and on the inlet/outlet tubesheets.

At this point, it looks like we just need a new bundle. Does anyone have any further suggestions?






Compositepro: Thanks for that - I've always heard economizer (https://en.wikipedia.org/wiki/Economizer). It might be a regional term. I've found various resources ("A Dictionary of Chemical Engineering" for example) that give definition to it, so it might just be a rose by another name.

RE: Oxidizer VoC Help

I'm not suggesting that economizer is a wrong term, it is just that it is a very broad term and therefore not very descriptive. Economizer is most often heard in the air conditioning industry and refers to using outside air for cooling when the conditions are suitable. It also does not distinguish between a recuperator or regenerator, which is important to your question. You could also have called it a heat exchanger.

The first thermal oxidizer I was involved in had blown-up before I started working for that employer, and there was a lawsuit. Solvent had been poured into a hot mixer and the flow to the incinerator went over LEL before the sensors and dampers could respond. A new one was supplied by the vendor at no charge. The new system also used a condenser to catch high solvent flows from the mixers.

Thank you for the pictures.

RE: Oxidizer VoC Help

To all,

Thanks OP and CompositePro for sharing. I'm a process engineer working in an EPC for thermal oxidizers as well. Though up to the last post, I'm still wondering what is the root cause of the poor destruction efficiency - is it flue gas leaking through the tubesheet? Since I don't see a direct relation with the observed cracks and DRE.

Hope to get your insights on this. It is really good sharing and educational, thanks in advance!

RE: Oxidizer VoC Help

(OP)
Wei, the economizer is after the combustion chamber (naturally). This economizer preheats the incoming process air (tubeside) that is laden with VoCs. The preheated process air then goes to the combustion chamber, then back to the economizer on the shell side. The tubesheet crack let a certain quantity of the process air bypass the combustion chamber and instead bypass straight to the shell-side, where it never reached sufficient temperature to achieve destruction.

A smoking gun, in retrospect, should have been the fact that DRE didn’t change one iota during the failed test when increasing combustion temp by 50 F.

UPDATE: after rebuilding the tube bundles, efficiency during the re-test was >99.5%.

RE: Oxidizer VoC Help

Great, nice to hear that! I had thought that you were using the economizer to preheat combustion air instead, as our company usually calls air preheaters "economizers" - I guess that's where the confusion stems from.

RE: Oxidizer VoC Help

A valved bypass across this new economiser may be a good idea so you can keep going even if the tubes leak or the tube sheet cracks some time in the future.

RE: Oxidizer VoC Help

Without the economizer (recuperator) a thermal oxidizer cannot be operated economically, as its purpose is to heat very large volumes of air to a high temperature. It would also probably violate the environmental permit.

RE: Oxidizer VoC Help

Agreed it wont be as economical as with the preheater, but it would help to tide over a rough spot if there is enough ullage on burner capacity to keep the combustion chamber at the required temp.

RE: Oxidizer VoC Help

(OP)
Georgeverghese,

We probably can't do that. Though the burner has enough BTU capacity to handle no preheat of process air,the piping and stack downstream are not designed to handle combustion chamber temps because they are not refractory-lined. Without the economizer, we'd have no means to cool the air before it hits that pipe.

We have two oxidizers installed in parallel. If we have a crack in one, we can run the other until the repair is made.

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